Salvation Brotherhood

"The creatures in the room grow afraid of you, you see that one creature is staring at you, with no fear nor hesitation, there is an iron fist emblem on his necklace. "You want to keep going down, do you? I am Vincent, member of the Savior's Brotherhood, can I help you? I can exchange runestones with you if you wish."

Salvation Brotherhood?

(Threat) Give me the rune stones now. You've seen what happened to these guys already."

The Threat is not a strength check. It failed with 14 Str. on Dellamorey. Based on the text when he leaves ('as if he read your mind, he quickly steps back') it could be an Int or Dex check or a combination of Str, Dex and Int. Luck was 365 so probably not based on that either. 2nd attempt worked at 14, 5, 5, 5.

This quest usually appears after the 10th floor, may require 150+ Luck score.